INTERMITTENT MISS

[thommo59]

200hp ProXs 2B783266
I have recently developed an intermittent miss (the hardest to pin point) I changed plugs and leads 40 hours ago along with its first service where fuel filters were replaced, yes on board and engine. I have since tried with the onboard filter but no difference.
So, get on plane, all good, say just cruising anything from 3600 to 4300 type rpm’s and engine starts to slow and both smart craft and vessel view show decline by 200-300rpm, no shaking or spluttering as such but almost like the handbrake is coming on. Then if I tap the pedal, a flat spot type miss happens and away we go if I give more throttle. This happens most everytime I move from spot to spot.
No codes or alarms, tried different fuel brands and 1 higher rating, still same. Haven’t pulled plugs yet as I expect to see no variance as it runs fine either side of the miss.
I know these are almost impossible to diagnose but hoping someone may have had the same and resolved there’s.

[VA Bass]

bad coil?? loose plug? like you said, it is hard to find that issue. Years ago I would have an old EFI do that and sometimes it just cut off while running. turned out to be bad altenator I think. I think checking those plugs would be a good start.

[Pulsare2020]

Do you have 2 fuel filters? The water separating filter on the engine and another one between the tank and engine? I suppose it worked fine from day 1 and after you replaced both it still did not operate as intended. Mercury says to only use one filter on the V8 engines but since both worked new on yours and did not after they were replaced it is unlikely that is the issue.

[thommo59]

It has worked fine but I bypassed the onboard filter to eliminate that and no difference

[EuropeanAM]

Filters and plugs would be a good starting point.

One other area to check is the fuel injector harnesses underneath the cover (covers the fuel rail and injectors). Sometimes we see harnesses rub against something, get pinched under the cover screw-boss, or simply have a bad spot in the harness lead. Can be difficult to find, but in many cases, a "wiggle test" (wiggling wires while running) may produce a consistent change in running quality if you flex the wiring near a particular injector (example: #7, bottom STB side). Causes a very wide array of symptoms and problems.

[thommo59]

Thank you I wil start working through it, I understand the frustration of an intermittent miss like this to diagnose, so rather than ask a dealer to spend hours chasing it, I’ll try first and at least eliminate what I can.

What about something like a demand sensor? Trying to associate something that reacts the moment it bump the throttle a little

[EuropeanAM]

While not impossible, the demand sensor has "dual" pots... one increasing and one decreasing voltage. The PCM is cross-checking them constantly, so if there is any consistency at all to something in the demand sensor, you will usually get a fault for it. Not always- but the lion's share of the time.

I'd start looking at the areas I mentioned earlier....

[thommo59]

Well, changed plugs and fuel filter, checked all wiring around injectors under the covers, it’s still doing it. Under power getting on plane is good, wot is good, it’s just that mid type cruising it just starts dropping rpm’s. You can hear it slowing, watch the rpm die off 200 -300 roughly, sits like this, I bump the throttle and get on it a bit and it hesitates a little then goes. Damn annoying.

[EuropeanAM]

Are you sure you're not experiencing the ARO (Advanced Range Optimization)? You're in the RPM band where it would be most likely to be active... changing the throttle setting would over-ride it for a while.

[thommo59]

Are you sure you're not experiencing the ARO (Advanced Range Optimization)? You're in the RPM band where it would be most likely to be active... changing the throttle setting would over-ride it for a while.

Nothings impossible I guess but I don’t think so. This starts dropping rpm, not holding it then gives a small sneeze type miss when I give it throttle, it feels like clearing a flat spot when it goes. It only started doing this approx 4 -6 weeks ago, prior it was fine. Called my local dealer but until and if it throws a code, there’s not much they can do.
Other than being annoying, my concern is when it does finally show itself and fail, it could be the worst timing

[EuropeanAM]

Might try venting your fuel cap on a clear day (slightly open but protected from moisture) to see if the problem continues to occur.

[thommo59]

Might try venting your fuel cap on a clear day (slightly open but protected from moisture) to see if the problem continues to occur.

Thankyou, I had tried opening the cap but then closing it before take off. I’ll try cracking it open a touch next time it does it, I’m open to any suggestions.